Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy and clean energy is increasing, and in a bid to meet the demand, the fields of electric power generation and electric energy storage using electric chemistry are most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and application thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
As the lithium secondary battery, lithium-containing cobalt oxide (LiCoO2) is mainly used, and in addition, the use of lithium-containing manganese oxide such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium-containing nickel oxide (LiNiO2) is also considered.
As the positive electrode active material of a lithium secondary battery, lithium-containing cobalt oxide (LiCoO2) is mainly used, and in addition, the use of lithium-containing manganese oxide such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium-containing nickel oxide (LiNiO2) is also considered.
Among the above-mentioned positive electrode active materials, LiCoO2 has excellent lifespan characteristics and charge/discharge efficiency and thus is most frequently used, but it has disadvantages that high temperature safety is lowered, and cobalt used as a raw material is expensive due to its resource restriction and thus there is a limit in price competition aspect.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 are advantageous in that they are excellent in thermal stability, and they are inexpensive and easy to synthesize, but there are problems that the capacity is small, the high temperature characteristics are poor, and the conductivity is low.
In addition, LiNiO2-based positive electrode active material is relatively inexpensive and shows battery characteristics such as high discharging capacity, but exhibits sudden phase transition of the crystal structure in accordance with volume change accompanying charge/discharge cycles. Further, there is a problem that the stability is abruptly lowered when exposed to air and moisture.
Therefore, recently, a lithium transition metal oxide in which a part of nickel is substituted with another transition metal such as manganese or cobalt has been proposed as an alternative material. However, such metal-substituted nickel-based lithium-transition metal oxides have an advantage in that they have relatively excellent cycle characteristics and capacity characteristics. However, even in this case, the cycle characteristics are drastically lowered when used for a long period of time, and problems such as swelling due to gas generation of the battery and deterioration of thermal stability due to low chemical stability are not sufficiently solved.
Therefore, there is a high need for a positive electrode active material capable of solving the thermal stability problem even while exhibiting improved capacity and output characteristics.